Suspended ceilings, also known as dropped ceilings, are often used to lower and modernize an existing ceiling, to cover a damaged ceiling or to hide exposed plumbing, wiring and joints. A metal network consisting of main runners, cross-pieces and L- or U-shaped edge molding is installed. Acoustical ceiling panels or metal pans are then inserted between the grids formed by the runners, cross-pieces and edge molding.
Sometimes a suspended ceiling is installed around a column or along a curved wall. Such an installation requires that the edge molding follow the curvature of the column or the wall. When the wall is serpentine (i.e., with compound curves), the installation can be quite complex.
It is not feasible to provide custom made curved edge molding because of the variety of shapes required and because curved walls and columns even built on the same plan vary to some extent. Ordinary edge molding can be notched with a hacksaw or sheet metal hand shear at intervals corresponding to the degree of curvature needed and then bent. This is a time consuming process and leaves an extremely unsightly installation as there are gaps between the individual notches as they open to accommodate the curvature of an outside curve. Around an inside curve, the tabs of metal between the notches are forced to interleaf and the molding tends to wrinkle.
The present invention relates to an improvement over the flexible edge molding described in U.S. Pat. No. 3,159,252 to Becker. The molding shown and described in the Becker patent is L- or U-shaped. The flanges of L- or U-shaped edge molding have wide square cut notches. Commercially available edge molding is formed from a flat strip of steel. The strip is notched along the sides to become flanges and a piece of plastic tape is wrapped around that edge to become the bottom flange. The molding is then placed in a bending brake having a press and a pivoted leaf. That side of the strip to become a web is held in the press while the leaf supports that part of the strip to become a flange. The base of the notch is about 1/8 inch from the middle of the bending radius. As the leaf pivots, deflection in the flange varies across the notched edge. The tabs between the notches are bent at a sharper angle than is the flange at the base of the notches. (Deflection in general is equal to Pl.sup.3 /mEI where P is force, l is the height of flange in the tab or base, m is the coefficient of deflection, E is the modulus of elasticity and I is the moment of inertia). In consequence, the base of the notch is bent at less than 90 degrees and shows as a relief in the tape.
If the notch is spaced more than 1/8 inch from the bend, the notch shows less in the tape but the flexible molding cannot be bent around as tight a curve. Commercially available flexible molding with the base of notch 1/8 of an inch from the bend can be bent to an outside radius of about 18 inches and an inside radius of about 36 inches. If the outside radius is much less than 18 inches, the flanges tend to lay out (i.e. the included angle between the flange and the web is made obtuse). When the molding lays out, the ceiling panels or pans do not lay flat on the flange and there is gap along the edge of the wall. On the other hand, if the flexible molding is used on a curve with an inside radius less than about 36 inches, the flange wrinkles between the tabs.